Such a solar cell relates to a solar cell with p- or n-type base.
The solar cell comprises a semiconductor substrate, e.g. silicon, which is covered on a back side by the p- or n-type back surface field (BSF) layer and on a front side by an emitter layer of a conductivity type opposite to that of the base layer. The front side of the substrate is arranged to be directed towards a light source during use of the solar cell.
In the prior art, the p- or n-type back surface field layer (BSF layer), is for example manufactured by a screen printing process (but also POCl3 face-to-face tube furnace diffusion, spray coating, spin coating, etc. can be used). During the screen printing process, a paste that comprises a p- or n-type element as dopant element is printed one-sidedly on the back side of the substrate. Next, a first diffusion process is carried out in which the substrate with the printed paste is heated to diffuse the p- or n-type element into the substrate to form the p- or n-type BSF layer. Such a heat treatment is typically carried out in an inline diffusion furnace. Subsequently, on the front side a layer of opposite conductivity type is formed by a second diffusion process, which is carried out in a second diffusion furnace. Alternatively, the emitter layer may be created first, followed by a creation of the BSF base layer.
The prior art manufacturing process has some disadvantages. The screen printing method to form the BSF layer on the back side may cause contamination of the front side of the substrate by parasitic diffusion or spill over of the dopant source to the front side. Parasitic diffusion of the dopant on the front side results in a non-uniform dopant concentration profile for the emitter layer which will adversely affect the efficiency of the solar cell. For example, shunt or high reverse current are typical effects. Furthermore, screen printing reduces the yield of the manufacturing process due to a relatively high chance of breakage of substrates.
Also, for n-type BSF layers that use phosphorus as n-type dopant, a one-sided diffusion results in a less efficient gettering of impurities from the substrate.